A typical scanning apparatus comprises a light-radiating device for moving along a scanning path to illuminate an object or document (for example, a piece of paper or a transparency) and projecting the scanned image of the scanned object through a focusing device to a receiving device (such as a charge-coupled device (CCD), or a contact image sensor (CIS)). However, the brightness distribution of light emitted from the light-radiating device to the surface of the scanned object will become uneven due to the properties of the light source. Consequently, the image quality projected to the image receiving device will be degraded.
Please refer to FIG. 1. In a typical CIS scanner, the light-radiating device (light source) comprises an LED 10 with a light guide 12. The LED 10 emits light, which transmits along the light guide 12 and is projected onto the scanned object. The light brightness distribution onto the text page of the scanned object 14 is shown in FIG. 1. In FIG. 1, Imax and Imin represent the maximum and the minimum brightness, respectively, whereas Im is defined as             I      max        +          I      min        2to represent the average brightness. The unevenness of light brightness distribution is defined as                     I        max            -              I        m                    I      m        ×  100  ⁢      %    .  
Currently, the unevenness of the light distribution on the surface of the scanned object may be resolved by configuring the geometric structure of the light guide 12 to make the light projected by the LED through the light guide more even (uniform). However, the result achieved is not always satisfactory. Another resolution is to first drive the light source in a CIS scanner to scan an even blank (white element). The scanned data then serves as a reference white. In later scanning of other documents, the scanned data of other documents will be compared with the corresponding value of the reference white to obtain image data approximate to the source documents. However, this method will reduce the image quality of the darker areas on the source documents.